Pyrometallurgical recovery of cobalt from waste reverbaratory furnace slag by DC plasma-ARC furnace technology

[摘要] ENGLISH ABSTRACT:Slag cleaning has become a common practice at many smelters in the nonferrous industryto maximize recovery of valuable metals. However, during the carbothermic reduction ofnonferrous slag to recover cobalt, in particular, iron is recovered predominantly. Highiron levels present a problem for the subsequent treatment of the alloy as it may increasethe solids loading to the filter and lead to increased reagent consumption during leaching.Finding an appropriate slag modifier that would selectively improve the recovery ofcobalt against that of iron to the metallic alloy can solve this problem. In the presentstudy the effects of lime (CaO), rutile (Ti02), and fluorspar (CaF2) on the recovery ofcobalt from waste nonferrous slag have been investigated under reducing conditions at1500°C. The selective recovery of cobalt compared to the recovery of iron at differentlevels of flux additions is discussed in this study, to show the selectivity of these fluxes. Itis also shown in the study that the recovery of cobalt does not only depend on the oxygenpartial pressure and temperature but on the slag composition as well.The slags used in the experiments were a) synthetic slag prepared from chemically purereagents and its composition was derived from that of the industrial nonferrous slagcomposition typical of reverbaratory furnace and b) actual slag obtained from an old slagdump situated on the Zambian Copperbelt region. The investigation has shown that Ti02addition leads to the most selective cobalt recoveries in all cases. On the other hand, bothCaO and CaF2 lead to higher overall cobalt recoveries. The effect of Ti02 on the slagchemistry leads to the formation of iron titanate compounds in the slag unlike CaO,which displaces FeO from the fayalitic slag and thus increases the activity of FeO inthe molten slag, which in tum affects the iron recoveries to the alloy product. CaF2 on theother hand, affects the fluidity of the slag leading to improved recoveries by improvedsettling of metals through the slag to the alloy product.A 44V/I100A DC-plasma arc furnace was used to reduce slag to recover cobalt at about13.5kV A power input. The major part of the study was conducted in a tube furnace andthe findings were applied to the extraction of cobalt from slag using the plasma-arcfurnace. It was found that the synthetic slag experiments could be used as a guide tounderstand the behaviour of cobalt during the carbothermic recovery of cobalt from silicasaturated fayalitic slags. Cobalt recoveries were higher in the DC furnace than the corresponding reduction experiments carried out in the tube furnace. However,significant amounts of silicon and carbon were detected in the metallic alloy product ofthe DC plasma-arc furnace.

[发布机构] Stellenbosch University